It can also indicate uncertainty, particularly if the lips are touched with the fingers. Lips can say a lot of things without words. This expression is also made when a person barely misses the target that he was trying to achieve. When the top lip is over the bottom lip, this may be linked with biting of the bottom lip, a common indicator that the person is feeling guilty about something. There isn't Facial expressions lips firmly pressed together better place to observe this gesture than an exam hall, where students often have no idea what the question paper is trying to say!